The present invention relates to ferrite particles for a bonded magnetic core and a ferrite resin composite which has a large magnetic permeability and an excellent fluidity.
Ferrite particles and a ferrite resin composite in the present invention are mainly used as a magnetic core material of an induction coil for various electronic machines such as a computer, communications apparatus and home appliances, and a magnetic core material of a transformer, electromagnetic wave absorption or shielding, etc.
As well known, a bonded magnetic core which is superior to a sintered magnetic core in dimensional stability, processability and resistance to brittleness, is advantageous in that a small or thin core is realizable and mass production of even cores having a complicated shape is easy. With the recent development of electronics, the demands for providing lighter-weight, miniaturization and higher-accuracy cores which are to be produced by making good use of these advantages has been increasing.
A bonded magnetic core is generally produced by kneading a magnetic material with a resin such as nylon and phenol resin, and molding the resultant mixture by compression molding or injection molding.
As the magnetic material, an oxide material such as Mn-Zn ferrite and Ni-Zn ferrite is used. Such an oxide magnetic material is generally obtained by mixing a main raw material such as Fe.sub.2 O.sub.3, ZnO and MnO or NiO in advance by wet or dry blending so as to have a desired composition, granulating the resultant mixture into particles having a diameter of about several mm to several ten mm, calcining the obtained particles and pulverizing the calcined particles into particles having an average particle diameter of several .mu.m to several hundred .mu.m.
A bonded magnetic core is required to have a magnetic permeability as large as possible. This demand has been increasing with the recent demand for a bonded magnetic core having a higher capacity.
It is known that a bonded magnetic core is composed of a magnetic material combined with a resin such as nylon and phenol resin, as described above, and that various properties, in particular, the magnetic permeability of the bonded core has a closer relation to and is more influenced by the properties of the magnetic material used in comparison with a sintered core. Therefore, in order to obtain a bonded magnetic core having a large magnetic permeability, it is advantageous to use ferrite particles having a large magnetic permeability as a magnetic material.
With the recent tendency toward bonded magnetic cores having a higher capacity, demands for smaller, thinner and complicated-molded products has been increasing. To satisfy such demands, it is important that a ferrite resin composite can sufficiently fill in all parts of the mold. For this purpose, the ferrite resin composite is required to have an excellent fluidity.
However, in the ferrite particles produced by mixing raw materials such as Fe.sub.2 O.sub.3, ZnO and MnO or NiO, granulating the resultant mixture into particles having a diameter of about several mm to several ten mm, calcining the obtained particles at a high temperature and pulverizing the calcined particles in accordance with the above-described conventional method, the crystal grains grow as large as several hundred .mu.m and become non-uniform. In addition, the crystal grain contains many pores. Due to the non-uiniform crystal grains and the presence of many pores, the magnetic permeability is lowered. As a result the obtained ferrite particles show a small magnetic permeability as magnetic powder. Furthermore, since the magnetic powder itself is angular particles by pulverization, the fluidity thereof is too poor for a suitable magnetic material for a bonded magnetic core.
A magnetic material suitable for obtaining a bonded magnetic core having a large magnetic permeability was conventionally proposed.
For example, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 55-103705 (1980), mixed ferrite particles consisting of particle groups having different particle sizes of from 100 .mu.m to 5 mm in diameter, for example, a large-particle group having a diameter of 400 .mu.m to 5 mm and a small-particle group having a diameter of 100 to 350 .mu.m are used as a magnetic material for obtaining a molded product (bonded core) having a large initial magnetic permeability. However, since the mixed ferrite particles contain particles having a large diameter such as 5 mm, they are not suitable as a magnetic material for a bonded magnetic core.
The magnetic permeability and the fluidity of the ferrite resin composite for producing a bonded magnetic core are mainly dependent on the properties of the ferrite particles which are mixed with base materials of a resin composite. The magnetic permeability of the ferrite resin composite has a tendency to be enlarged with the increase in the magnetic permeability of the ferrite particles mixed. The fluidity of the ferrite resin composite has a tendency to become more excellent as the average particle diameter of the ferrite particles mixed becomes smaller and the surfaces of the particles becomes smoother. The magnetic permeability of the ferrite particles has a close relation to the average particle diameter and, hence, the magnetic permeability of the ferrite resin composite is enlarged with the increase in the average particle diameter. On the other hand, when the average particle of the ferrite particles increases, the fluidity of the ferrite resin composite is deteriorated.
As to the relationship between the magnetic permeability and the average particle diameter of the ferrite particles obtained by the conventional method, when the average particle diameter is about 100 .mu.m, the magnetic permeability is about 18, and when the average particle diameter is about 200 .mu.m, the magnetic permeability is about 23.
Therefore, in order to obtain a ferrite resin composite having a large magnetic permeability and an excellent fluidity, the ferrite particles mixed are required to have an appropriate average particle diameter which produces a large magnetic permeability and does not obstruct the fluidity, in particular, an average particle diameter of not more than 200 .mu.m, and to have as smooth a surface as possible.
In the researches undertaken so as to provide ferrite particles which have a large magnetic permeability, an appropriate particle diameter and an excellent smoothness, the present inventors have noticed that in order to produce ferrite particles having a large magnetic permeability, it is necessary to obtain ferrite particle having uniform crystal grains and an appropriate grain size and containing no pore, and that in order to obtain such ferrite particles, it is important to use spherical granules for calcination which satisfy all the following conditions: (1) pores are easy to diffuse in the ferrite particles, (2) the ferrite particles are easy to balance with the calcination atmosphere, and (3) the ferrite particles easily receive heat uniformly. The present inventors have also paid attention to spray drying which is capable of granulation substantially in the form of a sphere. As a result, it has been found that by dispersing and mixing a mixed powder for producing ferrite particles consisting essentially of 47 to 58 mol %, calculated as Fe.sub.2 O.sub.3, of iron oxide or iron oxide hydroxide powder, 10 to 30 mol %, calculated as NiO, of nickel oxide powder and/or calculated as MnO, of manganese oxide powder and 15 to 40 mol %, calculated as ZnO, of zinc oxide powder into and with water containing 0.2 to 1.0 wt % of a surfactant based on the weight of the mixed powder for producing ferrite particles so as to prepare a water-dispersed slurry having a slurry concentration of 40 to 60 wt %, spray-drying the resultant slurry so as to obtain the granules having an average particle diameter of 25 to 180 .mu.m, and calcining the obtained granules at a temperature of 1100.degree. to 1350.degree. C., the obtained ferrite particles comprises crystal grains of 5 to 15 .mu.m in average diameter, and have an average particle diameter of 20 to 150 .mu.m and a magnetic permeability of not less than 24. The present invention has been achieved on the basis of this finding.